1. Field of the Invention
The present invention relates to a puffer type gas circuit breaker which is designed to compress SF.sub.6 gas in a puffer cylinder, to blow out the compressed SF.sub.6 gas toward a contact portion and to extinguish an arc generating at the contact portion, and, in particular, relates to a grounding switch gear device with an improved circuit breaking unit which is suitable for prolonging an interruptable arc time span.
2. Related Art
A conventional circuit breaker is designed to open-circuit a grounded power transmission line at the time of accident, in particular when lightning causes damage, in the power transmission line, and to interrupt the current flowing therethrough. However, if the circuit opening condition is maintained, interruption of the power supply is continued so that to avoid such a condition, the power transmission line is usually reclosed in about one second. However, in the case of a large electric power transmission system, because of a high transmission line voltage and large electrostatic capacity between transmission lines, the continuing time of a secondary arc is prolonged due to electrostatic induction from a sound phase after interrupting the failed phase. Such makes a high speed reclosing operation in about one second difficult, however, which is desired in view of an effective power transmission system operation.
In order to overcome the above problem through grounding both ends of the open-circuited fault phase by high speed automatic grounding devices HSGS as illustrated in FIG. 8, the secondary arc A.sub.2 is extinguished. Immediately thereafter, the high speed automatic grounding devices are open-circuited and the interrupted failed phase is reclosed. However, when a following accident is generated during the circuit opening operation of the high speed automatic grounding devices, a zero missing current condition in which the waveform of the AC current never passes through zero level as illustrated in FIG. 2 may occur, and such fault current in the zero missing current condition cannot be interrupted by the conventional circuit breaker.
It usually takes time of about four cycles until the zero missing current condition restores to a current condition having an ordinary AC waveform. The time of about four cycles corresponds to an arc extinguishing time of such following accident which is determined by the sum of a relaying time of two cycles from detection of the following accident to generation of an interruption command signal and an interrupting time of two cycles. Accordingly, when it is required to interrupt a current in the zero missing current condition with a gas circuit breaker, the gas circuit breaker is required to have a long interruptable time span of about four cycles.
However, in the conventional puffer type gas circuit breaker, all of the compressed gas in the puffer cylinder is blown out to the contact portion at the end of the circuit breaking operation, and it has been impossible to achieve such a long interruptable time span of about four cycles.
For fulfilling such necessity, a conventional countermeasure has been proposed to extend the distance between electrodes of the contact portion. JP-A-63-88723(1988) discloses an example of such a conventional countermeasure.
The structure of extending the distance between electrodes of the contact portion as referred to in connection with the conventional countermeasure increases the size and weight of the entire device which also increases the size and weight of the operating unit for the circuit breaker. As a result, the space at the installation site of the device is likely to be expanded.